The present invention relates to a flow rate measuring device for measuring a flow rate of a fluid and more particularly to a flow rate measuring device suited for measuring a flow rate of air taken into an internal combustion engine.
As a flow rate measuring device installed in an air intake passage of internal combustion engines as in automobiles, a thermal type has become a mainstream because of its ability to directly detect a mass flow rate. In the light of an increasing trend for stronger vehicle exhaust emission control and of a demand for improved mileage, a flow rate measuring device is being called for which can measure the flow rate with high precision even when there are intake air pulsation in the engine or when the increased pulsation cause an air flow from the engine toward an air cleaner (reverse flow). To make a precise measurement of air flow even in such states of the engine requires a flow rate measuring device with a response speed capable of following the pulsation and a direction detection function. One such known example is disclosed in JP-A11-248505.
The device disclosed in JP-A-11-248505, however, has a poor measuring accuracy for a forward flow since a fluid in a sub-passage does not flow smoothly in the forward direction. The flow conditions in the main passage and the sub-passage in the intake manifold change during an air flow pulsating state and during a backward flow state. Hence, the measuring errors increase when there is any pulsation or when a backward flow is produced. Particularly when a backward flow occurs, the measuring errors tend to increase because of a failure to pick up a sufficient amount of the backward flow. The intake air of a motor vehicle contains contaminants such as dust and oil, so the thin film portion of the flow rate measuring device is easily contaminated.